From: robfi680@gmail.com   
      
   Tonight’s quandry. Can a register file write history be used in   
   reservation stations to reduce the number of register read ports   
   required on the register file? And would it be smaller, faster than   
   adding register file ports?   
      
   There are a couple of cases:   
   1)	The operands were ready at time of queue.   
   2)	The operands were not ready at time of queue. That means there must   
   be an outstanding operation that at some point will update the register   
   file.   
      
   Operands that are valid at time of queue are stored in the re-order   
   buffer temporarily until dispatch. So, it should be possible to update   
   the operands in the reservation stations either from the instruction   
   dispatch, or later when the value is written to the register file.   
      
   An issue arises between queue and dispatch. If a value was updated in   
   the register file between queue and dispatch then it will not be latched   
   by the reservation station because it does not have the instruction yet.   
   If the reservation station has the instruction already waiting then the   
   register file write port can be used.   
      
   Rather than having more register file read ports, I was thinking of   
   having the reservation stations track (snoop) the register file write   
   history using a tapped shift register. The only time that must be   
   accounted for is between queue and dispatch. So, assuming that   
   instructions dispatched within a certain time frame, then using write   
   history could work.   
      
   I was thinking a 64 deep shift register with several taps could be used   
   to supply written values.   
      
   It is going to be at least two clock cycles between queue and dispatch   
   until operands/instructions are updated in the reservation stations. So,   
   the first tap would be at shift 3. Then maybe taps at 6, 12, 24, and 64   
   clocks.   
      
   There is not usually a huge amount of time between queue and dispatch,   
   unless the functional unit is busy. I think the longest operation is <80   
   clocks, so in worst case a 128-deep shift register should work.   
      
   LUTs can be turned into shift registers up to 64 bits IIRC. With four   
   64-bit write ports and 10 bit register tag about 300 LUTs would be   
   required for each shift level.   
      
   The history could be shared between several reservation stations.   
      
   --- SoupGate-Win32 v1.05   
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